1. Field of the Invention
The present invention relates to an image input apparatus that forms an image through a plurality of minute-image-formation optical systems.
2. Description of the Prior Art
In recent years, with the advent of highly information-oriented society brought by the development of communications media, there has been a keen demand for acquiring various kinds of information effectively and timely. Such information includes a very large proportion of image information, and therefore recording and saving of image information are essential for conducting advanced information processing activities. Conventionally, apparatuses such as photographic cameras and video cameras have been used for recording and saving image information. However, it is impossible to miniaturize beyond a certain limit such apparatuses by making their constituent components smaller, and therefore, to realize apparatuses so compact as to be portable all the time, it is necessary, and in fact expected, to develop a compact image input apparatus based on a novel construction.
An arrangement conventionally known to help miniaturize an image input apparatus makes use of a lens array composed of a plurality of microlenses combined together. This is an application of the compound-eye seen in the visual system of insects, and helps realize an optical system that occupies less volume, offers a wider angle of view, and is brighter than a “single-eye” image formation system.
As a conventional image input apparatus adopting this arrangement, for example, Japanese Published Patent Application No. S59-50042 discloses an image input apparatus composed of a microlens array, a pinhole array, and an image surface. Here, the microlenses form a reduced image of an object, and the pinholes, paired one to one with the microlenses, sample different parts of this reduced image, forming together an image of the object on the image surface.
As another example, Japanese Laid-Open Patent Application No. H5-100186 discloses an image input apparatus composed of a microlens array, a pinhole array, and a photosensitive element array. Here, the microlenses, pinholes, and photosensitive elements are grouped into units each composed of a microlens, a pinhole, and a photosensitive element, and the individual units convert optical signals received from different parts of an object (subject) into electric signals, that together represent image information.
As still another example, Japanese Laid-Open Patent Application No. H10-107975 discloses an image input apparatus in which a plurality of photosensitive elements are arranged for each of microlenses. Here, the image of an object formed by the microlenses is not sampled by pinholes that individually sample different parts thereof, but is directly read by a two-dimensionally extending photosensitive element array that yields signals of minute images. Here, an aperture stop is arranged on the object side of the photosensitive element array and the microlens array, and the individual microlenses observe different parts of the object with no overlap among the signals obtained.
However, the image input apparatuses disclosed in Japanese Published Patent Application No. S59-50042 and Japanese Laid-Open Patent Application No. H5-100186 mentioned above both have basically the same optical system, and suffer from unsatisfactory resolution in the image signals obtained. Specifically, in a lens-pinhole type compound-eye optical system, the number of units each including a microlens and a photosensitive element coincides with the number of dots, i.e. resolution, of the obtained image, therefore it is inevitable to increase the number of units to obtain a high-resolution image. However, even a microlens needs to have a certain size to function satisfactorily as a lens, and therefore, in this compound-eye optical system, it is impossible to increase beyond a certain limit the number of units even if the microlens are arranged in a closest packed structure. As a result, it is difficult to obtain a high-resolution image.
On the other hand, the arrangement disclosed in Japanese Laid-Open Patent Application No. H10-107975 mentioned above solves the problem of unsatisfactory resolution in the image signals obtained, but, to obtain the best optical characteristics in this optical system, it is necessary to arrange the microlens array and the photosensitive element array on a spherical surface having its center at the aperture stop, and thus this arrangement is not fit for the purpose of making the entire image input apparatus satisfactorily compact. In particular, the individual photosensitive elements need to be arranged at discrete locations on a curved surface, i.e. a spherical surface in this case. This makes the photosensitive elements difficult to arrange in desired positions, and thus makes the entire image input apparatus difficult to manufacture.
In another invention, no aperture stop is used, and a microlens array and a photosensitive element array are each arranged on a flat plane. However, here, to prevent interference between the optical signals from adjacent microlenses, the photosensitive element array needs to be arranged at a certain distance from the microlens array, and thus the photosensitive element array requires an unduly large fitting area. In addition, this arrangement is specialized for narrow object angles at the cost of optical characteristics, and therefore exhibits various drawbacks in practical use.